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If you only have one account for every git repository provider, for example, one for www.github.com and one for www.gitlab.com, you will need to add the following to your ssh-config file, ~/.ssh/config

host github.com
  HostName github.com
  IdentityFile ~/.ssh/github
  User git

host gitlab.com
  HostName gitlab.com
  IdentityFile ~/.ssh/gitlab
  User git

where the identity files contain your private key for each account. These can be generated by running

ssh-keygen

which will prompt you to specify a location and a passphrase:

Generating public/private rsa key pair.
Enter file in which to save the key (/home/rsoko/.ssh/id_rsa): 
Enter passphrase (empty for no passphrase): 
Enter same passphrase again: 

these are all optional, but to work with the above config we need to generate two keys, one located at ~/.ssh/gitlab and one at ~/.ssh/github. As for the passphrase, this can be left empty if you don’t want to enter a password every time you create an ssh connection (for example when pushing code to the remote repository).

check if everything works:

ssh -T git@gitlab.com

which gives me

Welcome to GitLab, @rsokolewicz!

multiple accounts per provider⌗

If you have multiple accounts on for example gitlab.com (e.g. one personal and one work), you can configure the ssh-config as follows

host gitlab-private
  HostName gitlab.com
  IdentityFile ~/.ssh/gitlab_private
  User git

host gitlab-work
  HostName gitlab.com
  IdentityFile ~/.ssh/gitlab_work
  User git

and follow the steps as before to generate gitlab_private and gitlab_work. The crucial step now is when setting up the remote-url of a git repository to map it to the correct entry in the above ssh configuration:

git remote set-url origin git@gitlab-private:repo.git

git@gitlab-private:repo.git has two important parts: git@...: and :....git. The first part that’s between git@...: corresponds to the name of the host that is specified in the ssh configuration. In the above case host gitlab-private. The name can be anything and is there just to map the correct ssh key to the corresponding repo. The second part :...git is the git-url to the repo.